Under high rainfall conditions on acid soils with shallow crop root systems the rate of N leaching is high. A simple model predicts nitrogen uptake efficiency as a function of the amount of rainfall in excess of evapotranspiration, rooting depth and degree to which N leaching is retarded in comparison with water transport. Field observations on acid soils in S.E. Nigeria and S. Sumatera (Indonesia) showed that this model should be amended to include the role of old tree root channels. Crop roots can follow these channels, which are coated with partly decayed organic matter, into the acid subsoil. Measurements of water infiltration with a Guelph permeameter and a methylene blue dye showed that such channels form the major infiltration sites during rainstorms. Implications for nitrogen use efficiency and cropping pattern are discussed.
Tag: leaching
Rooting depth in cropping systems in the humid tropics in relation to nutrient use efficiency
A simple model is presented for calculating the rooting depth of a crop or crop combination required to intercept leaching nutrients for different climatic and soil conditions. Important parameters in this model are the amount of water moving through the soil, which depends on excess of rainfall over evapotranspiration, and the apparent adsorption constant, which depends on the nutrient and soil type involved. Calculations for three time patterns of nutrient supply in relation to nutrient demand show moderate effects of the degree of synchronization on rooting depth required if a high interception fraction is desired. In shifting cultivation systems a deep-rooted fallow vegetation can recover nutrients leached to the subsoil during the cropping phase. The simple leaching model can indicate the combinations of climate zone and apparent adsorption constant for which such interception is possible. It appears that recovery of leached nitrate is only possible in the subhumid zone. In the humid tropics the continuous presence of a deep root system as part of the crop combination on the field is necessary to use nitrogen efficiently, except when acid soil conditions keep all nitrogen in the ammonium form or when an almost ideal synchronization exists of nitrogen supply and demand during the growing season. Some data are discussed on the root distribution of food crops and on the possibilities to establish a “safety-net” under the crops grown in alleys between deep-rooted hedgerow trees.
Modelling of planted legume fallows in western Kenya using WaNuLCAS. (II) Productivity and sustainability of simulated management strategies
Improved fallow is a technology that can help to raise agricultural productivity in systems of poor soil fertility and low financial capital. Models, once calibrated, can be used to investigate a range of improved fallow systems relatively quickly and at relatively low cost, helping to direct experimental research towards promising areas of interest. Six fallow crop rotations were simulated using the WaNuLCAS model in a bimodal rainfall setting in Kenya over a 10 year period: (A) alternating fallow and crop seasons, (B) one season fallow followed by three seasons crop, (C) one season fallow followed by four seasons crop, (DF) 13 seasons fallow periods followed by 35 seasons cropThe strategies were tested using a number of fallow growth rates, soil clay contents, and rainfall amounts to determine the interaction of fallow rotation and biophysical vari- ables on maize (Zea mays (L.)) yield and sustainability (organic matter, N2 fixation, leaching). The best simulated fallow strategies doubled maize yield compared to continuous maize over a 10 year period. Across all biophysical treatments strategy A and B of no more than three consecutive cropping seasons and of one consecutive fallow season yielded the most maize. This was because fallow benefits were largely due to the immediate fallow soil fertility benefit (IFB) rather than the cumulative benefit (CFB). The difference in yield between the two strategies was through a balance between (1) their interaction with the biophysical variables affecting accumulation of organic matter, hence increasing soil fertility and (2) the extra intrinsic soil fertility used for maize productivity by the inclusion of more cropping seasons within the rotation. We propose the following conceptual framework to manage fallows for maximum maize yield: when environmental factors are strongly limiting to fallow and crop growth then fallow strategy A would be the best strategy to employ (less risk but more labour) and when factors are less limiting then strategy B would be the best to employ.
Benefiting from N2 -fixation and managing rhizobia
Nodulation and nitrogen fixation in the Leguminosae (Fabaceae) family, classification of rhizobia, and quantification of N2 fixed by different legumes are reviewed. Some promising management options intended to improve N2-fixation by direct selection of rhizobia in soil or by screening and breeding of legumes for increased N2-fixation, are discussed. The effects of pruning, soil phosphorus and pH, available nitrogen, and interactions of nematodes and rhizobia on nodulation and N2-fixation are analysed. The fate of fixed nitrogen can be determined by the recovery efficiency of N and leaching and gaseous N losses.